A study in the Leningrad region assessed the population's immunity to SARS-CoV-2 during the COVID-19 epidemic. They found that 20.7% of the population had collective immunity. Children aged 1-6 and those over 70 had the highest immunity levels. Unemployed individuals showed higher immunity than civil servants and military personnel. Contact with COVID-19 patients increased the risk of infection by 1.5 times. After recovering from COVID-19, 82.1% developed antibodies. Among those with a previous positive PCR test, 82.8% had antibodies. 86.9% of those with antibodies were asymptomatic. These findings suggest a moderate level of immunity during the epidemic peak, with 18% not developing antibodies after infection. High rates of asymptomatic cases indicate a significant hidden spread of the virus. These results are crucial for planning preventive measures, including vaccination, and predicting disease spread.